Transverse ventilation is a new type of ventilation for large warehouse, which effect of water retention and cooling is rarely studied either at home or abroad. Based on the theory of heat and mass transfer in porous medium, the temperature variation and water migration in the grain bulk during transverse ventilation in different air temperature and humidity conditions is predicted by numerical simulation. The optimal ventilation condition of water retention and cooling is obtained by analyzing the distribution of temperature and moisture in the grain bulk in the condition of different air temperature and time. The result provides theoretical guidance for reasonable operation in mechanical ventilation.